Carbohydrates
Dr. Açelya Yılmazer
Carbohydrates
• Named so because many have formula Cn(H2O)n
• Produced from CO2 and H2O via photosynthesis in plants
• Range from as small as glyceraldehyde (Mw = 90 g/mol) to as large as amylopectin (Mw = 200,000,000 g/mol)
• Fulfill a variety of functions including:
– energy source and energy storage
– structural component of cell walls and exoskeletons – informational molecules in cell-cell signaling
• Often covalently linked with proteins to form glycoproteins and proteoglycans
Aldoses and Ketoses
An aldose contains an aldehyde functionality
A ketose contains a ketone functionality
Enantiomers
• Enantiomers: stereoisomers that are non-superimposable mirror images
• In sugars that contain many chiral centers, only the one that is most distant from the carbonyl carbon is designated as D or L
• D and L isomers of a sugar are enantiomers
– e.g. L and D glucose have the same water solubility
• Most hexoses in living organisms are D stereoisomers
• Some simple sugars occur in the L-form, such as L- arabinose
Diastereomers
• Diastereomers: stereoisomers that are not mirror images
• Diastereomers have different physical properties
– e.g. water solubilities of threose and erythrose are different
Drawing Monosaccharides
• Chiral compounds can be drawn using perspective formulas
• However, chiral carbohydrates are usually represented by Fischer projections
• Horizontal bonds are pointing towards you;
vertical bonds are projecting away from you
Epimers
• Epimers are two sugars that differ only in the
configuration around one carbon atom
Structures to Know
• Ribose is the standard five-carbon sugar
• Glucose is the standard six-carbon sugar
• Galactose is an epimer of glucose
• Mannose is an epimer of glucose
• Fructose is the ketose form of glucose
Hemiacetals and Hemiketals
• Aldehyde and ketone carbons are electrophilic
• Alcohol oxygen atom is a nucleophilic
• When aldehydes are attacked by alcohols, hemiacetals form
• When ketones are attacked by alcohols,
hemiketals form
Cyclization of Monosaccharides
• Pentoses and hexoses readily undergo intramolecular cyclization
• The former carbonyl carbon becomes a new chiral center, called the anomeric carbon
• The former carbonyl oxygen becomes a hydroxyl group; the position of this group determines if the anomer is or
• If the hydroxyl group is on the opposite side (trans) of the ring as the CH2OH moiety the configuration is
• In the hydroxyl group is on the same side (cis) of the ring as the CH2OH moiety, the configuration is
Pyranoses and Furanoses
• Six-membered oxygen-containing rings are called pyranoses
• Five-membered oxygen-containing ring are called furanoses
• The anomeric carbon is usually drawn on the
right side
Chain-ring Equilibrium and Reducing Sugars
• The ring forms exist in equilibrium with the open- chain forms
• Aldehyde can reduce Cu
2+to Cu
+(Fehling’s test)
• Aldehyde can reduce Ag
+to Ag
0(Tollens’ test)
• Allows to detect reducing sugars, such as glucose
The Glycosidic Bond
• Two sugar molecules can be joined via a
glycosidic bond between an anomeric carbon and a hydroxyl carbon
• The glycosidic bond (an acetal) between
monomers is less reactive than the hemiacetal at the second monomer
• The disaccharide formed upon condensation of
two glucose molecules via 1 4 bond is called
maltose
Nonreducing disaccharides
• Two sugar molecules can be also joined via a glycosidic bond between two anomeric carbons
• The product has two acetal groups and no hemiacetals
• There is no reducing ends, this is a nonreducing disaccharide
• Trehalose is a constituent of hemolymph of insects
• Provides protection from drying
– Resurrection plant (> 15 yrs)
Polysaccharides
• Natural carbohydrates are usually found as polymers
• These polysaccharides can be – homopolysaccharides
– heteropolysaccharides
• Polysaccharides do not have a defined molecular weight.
– This is in contrast to proteins because unlike proteins, no template is used to make
polysaccharides
Glycogen
• Glycogen is a branched homopolysaccharide of glucose
– Glucose monomers form (1 4) linked chains – Branch-points with (1 6) linkers every 8-12
residues
– Molecular weight reaches several millions
– Functions as the main storage polysaccharide in
animals
Starch
• Starch is a mixture of two homopolysaccharides of glucose
• Amylose is unbranched polymer of (1 4) linked residues
• Amylopectin is branched like glycogen but the branch-points with
(1 6) linkers occur every 24-30 residues
• Molecular weight of amylopectin is up to 200 million
• Starch is the main storage homopolysaccharide in
plants
Metabolism of Glycogen and Starch
• Glycogen and starch often form granules in cells
• Granules contain enzymes that synthesize and degrade these polymers
• Glycogen and amylopectin have one reducing end but many non-reducing ends
• Enzymatic processing occurs simultaneously in
many non-reducing ends
Cellulose
• Cellulose is a branched homopolysaccharide of glucose
– Glucose monomers form (1 4) linked chains – Hydrogen bonds form between adjacent
monomers
– Additional H-bonds between chains
– Structure is now tough and water-insoluble
– Most abundant polysaccharide in nature
– Cotton is nearly pure fibrous cellulose
Cellulose Metabolism
• The fibrous structure, and water-insolubility makes cellulose a difficult substrate to act on
• Fungi, bacteria, and protozoa secrete cellulase, which allows them to use wood as source of glucose
• Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze (1 4) linkages
• Ruminants and termites live symbiotically with a microorganisms that produces cellulase
• Cellulases hold promise in the fermentation of biomass into biofuels
Chitin
• Chitin is a linear homopolysaccharide of N-acetylglucosamine – N-acetylglucosamine monomers form (1 4) linked
chains
– Forms extended fibers that are similar to those of cellulose – Hard, insoluble, cannot be digested by vertebrates
– Structure is now tough but flexible, and water-insoluble – Found in cell walls in mushrooms, and in exoskeletons of
insects, spiders, crabs, and other arthropods
